A compandor system is a data transmission system that includes a signal compressor coupled to one end of a transmission channel, at the other end of which a signal expandor is located. The compressor compresses the amplitude of a signal in such a way that all contributions to the compressed signal lie above the noise level inherent to the transmission channel, but do not run into the maximum dynamic range limits of the channel. Clipping and distortion are thereby avoided. At the receiving end of the channel, the expandor performs the inverse operation on the transmitted signal to restore or adapt it for further processing. Thus, the signal is accommodated to the transmission channel's limitations. In, for instance, tape decks, cordless telephones and wireless microphones, the compandor performs noise reduction. For signal level control, the compandor plays an important part in, for example, electronic organs, modems and mobile telephone equipment.
The idea of signal companding originated at Bell Laboraties in the forties. Compandors then included transformers, hybrids and passive components. In the midst of the seventies Signetics Company, a part of North American Philips Corporation, was the first company to manufacture an integrated circuit compressor and expandor. Nowadays, a wide variety of integrated compandors are available. See, for in, "AN176 Compandor Cookbook", Linear Data Manual Volume 1: Communications, Signetics Co., 1989, pp. 4-334/4-340.
A prior integrated compressor circuit and a prior art integrated circuit of the considered kind may each include the followingparts: a voltage-to-current converter for converting an input voltage into a control current; a current multiplier coupled to the converter for supplying an output current substantially proportional to both the input current and the control current and substantially inversely proportional to a reference current; and a differential amplifier coupled to the multiplier for receiving at an input a voltage indicative of the output current.
In both circuits, the voltage-to-current converter has an input coupled to a first node through a first resistor. The converter provides the control current at a control output. The control current is indicative of the voltage at the first node. The control current is, for instance, proportional to the time-averaged value of the voltage at the first node and inversely proportional to the value of the first resistor. The current multiplier has an input coupled to the first node through a second resistor for receiving the input current which is proportional to the voltage at the first node and inversely proportional to the value of the second resistor. The control output of the voltage-to-current converter is coupled to the current multiplier. The current multiplier supplies an output current to a second node.
The compressor circuit and the expandor circuit mainly differ in the way the amplifier is coupled to the other parts. In a compressor circuit, an inverting input of the amplifier is coupled to the second node and to the system's signal input through a further resistor. A non-inverting input of the amplifier is coupled to a reference node for receiving a reference voltage, and an amplifier output is coupled to the first node and to the system's channel. In the expandor, the inverting input is coupled to the second node and to the amplifier output through a further resistor, the non-inverting input is coupled to a reference node for receiving a reference voltage, and the first node is coupled to the channel.
One of the quantities that determine the operation of the compressor or expandor is the unity-gain level. The unity-gain level is the signal level that passes unaltered through the compressor or the expandor, as the case may be. Another quantity that is an indication of the system's performance is the dynamic range. This is the range of signal levels that can be processed without running into problems involving noise and signal distortion caused by clipping. The dynamic range in general is the largest when the quiescent operating voltage (bias voltage) is substantially halfway between the supply voltages of the compressor or the expandor.
Integrating a compandor leads towards compromises in the performance of the device, as a consequence of the particular integrating technology used and the particular application intended of the compandor. Therefore, the performance of the known circuits is optimized with regard to the dynamic range, set by the supply voltage, and the noise characteristics of the particular transmission channels used. This results, among other things, in a fixed compandor's unity gain level dedicated to a particular application.
For instance, the Signetics compandor circuits NE570 and NE571 have unity gain levels of 776 mV rms. The Signetics compandor circuit NE575 has a unity gain level of 100 mV rms, and the Signetics compandor circuit NE5750 has a unity gain level of 77.6 mV rms. These fixed unity gain levels are now well-established standards.
The integrated circuit's fixed unity gain level is a trade-off between device performance and technology available in view of the device parameters to be optimized. The known devices permit some marginal calibration of the gain by means of adding external resistors, to be arranged in series with the integrated resistors. However, this either affects the dynamic range of the compressor or the expandor, or deteriorates the unity gain level even further due to the external resistors' additional temperature or process parameter spread dependencies. Furthermore, changing the resistor values by arranging external resistors in series with the integrated resistors only allows the aggregated resistor values to be increased, whereas a decrease cannot be attained. More importantly, the very limited calibration range is totally insufficient for purposes that require non-standard unity gain levels.
In the prior art, extending the application of these high-quality circuits beyond the predetermined unity gain levels has generally been accomplished by adding level-shifters to fixed-unity-gain circuits, for level-shifting the signal prior to compressing and/or after expanding. This, however, involves additional circuitry for the auxiliary operations and, as a consequence, leads to an increase in power consumption, noise contribution and cost. Especially in the field of portable equipment, e.g. for consumer applications, these are serious drawbacks.